Ice-based thermal energy storage device

ABSTRACT

Disclosed is a heat exchange device including a first thermally conductive tube that is hollow over its length, a second thermally conductive tube that is hollow over its length, and including a thermally conductive fin, in which the fin extends lengthwise along the first tube, the fin extends lengthwise along the second tube and the fin extends width-wise between the first tube and the second tube.

The invention relates to the technical field of heat exchangers, and in particular heat accumulators that transfer heat between a refrigerant and an external phase change or icing-based material, such as a mixture of water and ice at 0° C., via a heat-conducting material, such as a metal, separating the refrigerant and the phase change material. Two significant parameters for the cost of an exchanger of this type are the volume of heat-conducting material and the volume of refrigerant used, with respect to the energy stored.

Two major families of heat accumulators by phase change are known from the prior art:

-   -   tubular: these accumulators are made up of a multitude of tubes         that are hollow over their length and parallel to one another,         each tube being filled with a pressurized refrigerant in outer         contact with an icing-based medium. Such tubes are made         primarily from a heat-conducting material, typically metal, in         particular steel, aluminum, and more rarely, copper.     -   Some tubular accumulators are made up of plastic tubes. Despite         plastic having a thermal resistance greater than that of metals,         their lower cost makes it possible to offset this handicap by         larger exchange surfaces. One variant of these batteries         consists of a single very long tube, arranged in a spiral or         helix or coil.     -   One variant of these accumulators consists of circulating a         secondary refrigerant in the tubes, typically glycol water,         instead of the refrigerant.     -   flat: these accumulators are made up of a multitude of flat         exchangers, parallel to one another. These exchangers are in         turn made up of 2 thin flat metal plates, hermetically joined by         their edge and spot welded to one another, arranged at regular         intervals with a matrix distribution (in rows and columns), when         the plates are observed on their largest face. During the         manufacture of these exchangers, an inner space is created by         injecting a gaseous or liquid fluid, under high pressure, until         the plates are permanently deformed, giving the plates a         pillow-like appearance, hence their name “pillow plate”.     -   The inner space of each exchanger is filled with pressurized         refrigerant and the outer surface of each exchanger is in         contact with a medium to be cooled until icing/ice-formation.     -   Such pillow plates are made up of sheets made from a         heat-conducting material, typically a metal, in particular steel         (generally stainless) and aluminum.     -   Some flat accumulators are made from plastic, again with a         thermal resistance of the plastic greater than that of metals,         but at a lower cost that makes it possible to offset this         handicap by greater exchange surfaces.     -   One variant of these accumulators also consists of circulating a         secondary refrigerant in the exchangers, typically glycol water,         instead of refrigerant.

The operation of these phase change heat accumulators, whether tubular or flat, consists of storing cold as follows:

-   -   producing frigories by evaporating the refrigerant contained in         the inner volume of each exchanger,     -   initially: transporting the frigories through the material of         the exchanger into contact with the liquid phase change         material,     -   then later: transporting the frigories through the material of         the exchanger and the thickness of ice into contact with the         still-liquid phase change material,     -   solidifying the still-liquid phase change material that is in         contact with the ice by transferring the frigories from the ice         to the material.

One of the features shared by all of the heat exchangers is that the evaporation temperature of the refrigerants drops as the thickness of ice increases. Indeed, the increase of the thickness of ice over time causes an increase in the thermal resistance, which opposes the flow of frigories between the refrigerant and the still-liquid phase change material. To combat this resistance, at a constant heat exchange power, the refrigerant naturally decreases its evaporation temperature over time.

In the prior art, a very different behavior of tubular heat accumulators is observed relative to flat or plate heat accumulators, which leads to major differences in terms of energy:

-   -   tubular accumulators: the cold generated within the exchanger by         the evaporation of the refrigerant causes anisotropic ice         formation, i.e., ice growth is concentric to the tubes or         perpendicular to their length or radial with respect to the         tubes. This anisotropic characteristic of the conduction causes         a drop in the evaporation temperature of the refrigerant as a         function of time with a steep slope, of about 2° C./hour.     -   flat batteries: the cold generated within the exchanger by the         evaporation of the refrigerant causes ice formation normal or         perpendicular to the two outer surfaces. This characteristic         causes a drop in the evaporation temperature of the refrigerant         as a function of time with a very gentle slope, of about 0.3°         C./h, or about 7 times less than tubular accumulators.

The study of the behavior of a tubular accumulator and a flat accumulator, the exchangers of which are manufactured with the same quantity of thermally conductive material, leads to the following situation:

The value “dT”, representing the temperature deviation between the refrigerant, boiling within a thermal accumulator operating at a constant power, and the phase change temperature of a phase change material, is equal, after operating for 4 h30, to:

-   -   tubular accumulator: dT=9.4° C.     -   flat accumulator: dT=1.4° C.

Yet it is commonly recognized by the profession that a gain of 1° C. on the evaporation temperature of a refrigerating machine corresponds to an increase of its energy efficiency of 2.5 to 3%. In the case at hand, the difference of 8° C. corresponds to a difference of 20 to 24% in terms of energy efficiency.

But this strong improvement in the energy efficiency has a cost: flat batteries are more expensive to produce (very long welding lengths) and require a significant volume of refrigerant.

There is therefore an unmet need in the prior art for a device for a heat exchanger, imparting a thermodynamic performance close to that of flat exchangers, using a volume of refrigerant close to or less than that of tube exchangers, with an identical volume of thermally conductive material, and the welding lengths of which are close to or less than those of tube exchangers.

Furthermore, tubes with fins or radiators are known in the prior art, for discharging, by fins, the heat produced by tubes filled with a heat transfer fluid and bathing in air, by convection in air. However, the fins of these tubes are used in the prior art to move the heat away from a tube as much as possible. From there, the fins are not considered suitable for use with matrixes of tubes, which are necessarily as close as possible in a heat accumulator. Indeed, the heat moved away by the fins is presumed to be communicated to the adjacent fins and tubes, which then decreases the efficiency of the assembly and its radiator function.

In general, in the prior art, an unfavorable prejudice exists against the use of fins in a heat accumulator, including against ice formation, the heat function of the fins for moving the heat away from the tubes being incompatible with the closeness of the tubes necessary in a heat accumulator to maximize icing in a given enclosure volume of the heat accumulator.

In this context, the invention is a heat exchange device, characterized in that it comprises a first thermally conductive tube that is hollow over its length, a second thermally conductive tube that is hollow over its length, and a thermally conductive fin, in which the fin extends lengthwise along the first tube, the fin extends lengthwise along the second tube and the fin extends width-wise between the first tube and the second tube.

In variants of the device:

the fin comprises a first thermally conductive half-fin, which extends between the first tube and a first edge and in which the fin comprises a second thermally conductive half-fin, which extends between the second tube and a second edge.

the thickness of the fin is less than the width of the first tube and is less than the width of the second tube.

the fin is flat.

the first tube is parallel to the second tube.

the first tube is straight over its length and the second tube is straight over its length.

the first tube is made from aluminum, the second tube is made from aluminum and the fin is made from aluminum.

the second half-fin extends width-wise between the first half-fin and the second tube.

the first edge of the first half-fin is in contact with the second edge of the second half-fin.

The invention also relates to a method using the device as defined above, characterized in that it comprises the following steps:

evaporating a refrigerant in the first tube and in the second tube,

solidifying a liquid material on the first tube, on the second tube and on the fin.

In a variant of the method, the liquid material is a water and the refrigerant has an evaporation temperature lower than the icing temperature of water.

The various embodiments of the invention are described for the references to numbers in parentheses, in connection to a list of figures provided with the present application, in which:

FIG. 1 shows a device according to the invention with a continuous fin joining two hollow tubes.

FIG. 2 shows a device according to the invention with a discontinuous fin between the two hollow tubes, made up of two half-fins extending between the tubes.

Different embodiments of the invention are described below.

In a first embodiment, the heat accumulator is made up of tubes arranged in parallel in a plane. A flat fin that is thin compared to the width of the tubes, which is their diameter for cylindrical tubes of revolution, is arranged between the tubes in the plane of the tubes. The tubes and the fin are arranged in a shell containing water at atmospheric pressure. A refrigerant, which is for example a refrigerant of type R134a, the evaporation temperature of which is below the freezing temperature of water, or 273° Kelvin, in a given pressure range, is circulated by a refrigeration machine comprising a compressor, able to operate in this pressure range.

FIG. 1 thus shows a characterizing part of the heat accumulator above in which a first tube (1) and a second tube (2) are connected along their length by a continuous flat fin (3) that extends over the entire width between the first tube (1) and the second tube (2).

The first tube (1), the second tube (2) and the fin (3) are for example made from aluminum or a thermally conductive material and make it possible to obtain, by extrusion, a homogeneous part integrally molded in one piece including the first tube (1), the second tube (2) and the fin (3) in a continuous assembly.

Preferably, the first tube (1) and the second tube (2) have the same length, the same width and the same thickness and are cylindrical and of revolution around a first axis for the first tube (1) and a second axis for the second tube (2).

The fin preferably has a minimal thickness allowed by the extrusion method, i.e., between 1 mm and 1.5 mm, but it also has a thickness smaller than the width or diameter of the tubes and preferably about the thickness of the tubes, here 1.5 mm. It also has a width equal to the distance between the tubes and this distance between the tubes is chosen to be greater than ten times the width of a tube. For example, a width of the tubes of 8 mm and a thickness of the tubes of 1.5 mm can be chosen, and a distance between the axes of the tubes of 100 mm, giving a fin width of 92 mm.

In all cases, a criterion of the invention can be verified for any geometry of tubes and fin. This criterion can be obtained experimentally by introducing the device according to the invention into a heat accumulator and tracing, as a function of time at a constant refrigeration power, the drift of the evaporation temperature of the refrigerant in the accumulator, or “dT” as previously indicated. For example, a pressure gauge can be used conventionally and the temperature of the fluid, such as the R134a, can be deduced by the curve of the saturated vapor pressure as a function of the temperature of the mixture.

For a system according to the invention, one can see that the curve representing the “dT” as a function of time has a slope very close to that of flat exchangers, and therefore very different from the slope of tube exchangers.

The system according to the invention therefore does not work like a tube system, but like a plate system in terms of icing. One can therefore deduce that the system according to the invention allows anisotropic icing, like flat exchangers, while making it possible to use a much smaller volume of refrigerant than that of a tube system, and a fortiori the volume of refrigerant of a flat system.

Several variants of the system of FIG. 1 can be considered, in particular the fin can have a different shape from a plane, outside the plane of the axes of the tubes, the tubes can be not strictly parallel and can be straight or rectilinear or curved with different shapes over their length, with a circular or elliptical or even rectangular section.

The material of the tubes and the fin can be a thermally conductive material, other than aluminum, such as a metal, in particular stainless steel or copper.

The variants will be favored that use the least amount of material and the least amount of refrigerant, while continuing to operate as a plate system, in terms of the evaporation temperature drift over the course of icing.

The fin thermally connecting the tubes changes its behavior during icing and makes it possible to place a small number of tubes to thermally re-create the operation of a plate.

Furthermore, practically, the first tube (1) has, diametrically opposite the fin (3) in the plane of the tubes, a third half-fin (4), with the same thickness as the fin (3) and a width for example equal to half the width of the fin (3).

Likewise, the second tube (2) has, diametrically opposite the fin (3) in the plane of the tubes, a fourth half-fin (5), with the same thickness as the fin (3) and a width for example equal to half the width of the fin (3).

In a second embodiment of the invention, which is also the preferred embodiment for the invention, the fin is made discontinuously between the first tube (1) and the second tube (2) in the form of a fin cut into two half-fins along its length: a first half-fin (6) starting from the first tube (1) up to a first edge and a second half-fin (7) starting from the tube (2) up to a second edge.

The cutting of the fin into two half-fins can be done along an edge with any shape without going beyond the teaching of the present application. However, a straight shape of the edges is particularly advantageous. Indeed, the structure of the invention in this second embodiment can be made from a single extruded piece comprising a central tube and two lateral half-fins in a same plane. It is thus possible to make the device illustrated in FIG. 2 by taking two of these parts extruded parts and making their tubes parallel and their fins coplanar in the plane of the tubes.

When the facing edges of two half-fins mechanically touch, one thus obtains the thermal equivalent of an extruded part with a width equal to that of a part multiplied by the number of parts used.

When the facing edges are separated, one obtains icing between the edges and a practically unchanged operation relative to the joint plates.

Outside a transitional state for the first embodiment and the second embodiment, the flow of frigories in the middle of the fin (3) is nil and the flow of frigories between the first half-fin (6) and the second half-fin (7) is nil.

By comparison with the existing accumulators, one can see the following features of the second embodiment for joined facing edges.

Battery Second Type Tubular Flat embodiment Units “dT” at 4h30 9.4 1.4 2.9 ° C. Length of welds 5 10 1 meter Quantity of fluid 2 4 0.4 liter Pressure holding 50 10 50 Bar

Synthetically, it emerges that the battery according to the second described embodiment is the least expensive of the three to manufacture (5 to 10 times less welding), that it is also the least expensive to use (energetically conservative and requires 5 to 10 times less refrigerant than the other two techniques), for resistance to pressure equal to or greater than the other two.

To obtain this comparative table, the modeling and simulation hypotheses are as follows:

-   -   equivalent thermal power for all three types of battery.     -   identical quantities of thermally conductive material for the         heat for all three types of accumulator.

One skilled in the art may, by a simple operations or by applying the “dT” criterion proposed in the first embodiment and in the second embodiment, verify, for a structure of these first and second embodiments, whether the behavior of the selected geometric structure is indeed a thermal behavior similar to a pillow plate or plate.

In particular, for a structure operating according to the invention, the curve of the evolution of “dT” as a function of time or over time, will have a slope very close to the same curve drawn for a flat exchanger, with a practically constant shift. This shift will be about 1.5° C. after 4 h30.

In the device of FIG. 2, the third half-fin (4) is diametrically opposite the first half-fin (6) on the first tube (1) and the fourth half-fin (5) is diametrically opposite the second half-fin (7) on the second tube (2). One can indeed see in this way that this embodiment can be obtained by arranging, mechanically fixed in the same plane, two identical extruded parts, one made up of the first tube (1), the first half-fin (6) and the third half-fin (4) and the other of the second tube (2), the second half-fin (7) and the fourth half-fin (5).

For example, the tubes have an outer diameter of 8 mm and an inner diameter of 5 mm for a thickness of 1.5 mm. Each half-fin has a width of 46 mm, for a width of about 100 mm of each extruded part and a thickness of the half-fins of 1.5 mm. The edges of facing half-fins are preferably joined. Like for the first embodiment, the distance between the tubes, here twice 46 mm for joined fins, is chosen to be greater than ten times the width of a tube, here 8 mm.

The same variants as in the first embodiment can be considered on the shape of the tubes and half-fins and their component material, which is typically a metal suitable for extrusion.

Furthermore, in this embodiment, it is desirable to place the facing edges of the half-fins in mechanical contact, to maximize the storage energy density of the accumulator.

For this embodiment, it is possible to consider half-fins with different widths.

Typically in the described embodiments, the ratio of the inner surfaces, presumed to be smooth, in contact with the refrigerant and the outer surfaces in contact with the phase change material is greater than 10. This constitutes an unusual anisotropy in the prior art for a tube with lateral fins. However, despite such an anisotropic ratio, thermal simulations using methods known from the prior art show that 3 times more heat is exchanged via the fin or the half-fins than via the tube, which indeed validates the influence of the fin on the icing.

For all of the described embodiments, it will be possible to use tube or fin or half-fin structures in a refrigeration machine as evaporator, to cool and solidify, using a refrigerant evaporating in the tubes, a liquid material, preferably calm or immobile, surrounding the tubes and the fins or half-fins.

The invention is open to industrial application or useful in the field of heat accumulators, transferring heat between two phase change materials and in particular for storing energy in the form of ice from freshwater or saltwater or brackish water.

Throughout the application, the addition of a “frigorie” to a thermodynamic system will be defined as the removal of a calorie from that thermodynamic system. 

1. A heat exchange device, comprising a first thermally conductive tube (1) that is hollow over a length of said first thermally conductive tube, a second thermally conductive tube (2) that is hollow over a length of said second thermally conductive tube, and a thermally conductive fin (3), in which the fin (3) extends lengthwise along the first tube (1), the fin (3) extends lengthwise along the second tube (2) and the fin (3) extends width-wise between the first tube (1) and the second tube (2).
 2. The device according to claim 1, wherein the fin comprises a first thermally conductive half-fin (6), which extends between the first tube (1) and a first edge and in which the fin comprises a second thermally conductive half-fin (7), which extends between the second tube (2) and a second edge.
 3. The device according to claim 1, wherein the thickness of the fin (3) is less than the width of the first tube (1) and is less than the width of the second tube (2).
 4. The device according to claim 1, wherein the fin (3) is flat.
 5. The device according to claim 1, wherein the first tube (1) is parallel to the second tube (2).
 6. The device according to claim 1, wherein the first tube (1) is straight over the first tube's length and the second tube (2) is straight over the second tube's length.
 7. The device according to claim 1, wherein the first tube (1) is made from aluminum, the second tube (2) is made from aluminum and the fin (3) is made from aluminum.
 8. The device according to claim 2, wherein the second half-fin (7) extends width-wise between the first half-fin (6) and the second tube (2).
 9. The device according to claim 2, wherein the first edge of the first half-fin (6) is in contact with the second edge of the second half-fin (7).
 10. A method using the device according to claim 1, the method comprising: evaporating a refrigerant in the first tube (1) and in the second tube (2), solidifying a liquid material on the first tube (1), on the second tube (2) and on the fin (3).
 11. The method according to claim 10, wherein the liquid material is water and the refrigerant has an evaporation temperature lower than the icing temperature of water.
 12. A method using the device according to claim 2, the method comprising: evaporating a refrigerant in the first tube and in the second tube, solidifying a liquid material on the first tube, on the second tube and on the fin.
 13. A method using the device according to claim 3, the method comprising: evaporating a refrigerant in the first tube and in the second tube, solidifying a liquid material on the first tube, on the second tube and on the fin.
 14. A method using the device according to claim 4, the method comprising: evaporating a refrigerant in the first tube and in the second tube, solidifying a liquid material on the first tube, on the second tube and on the fin.
 15. A method using the device according to claim 5, the method comprising: evaporating a refrigerant in the first tube and in the second tube, solidifying a liquid material on the first tube, on the second tube and on the fin.
 16. A method using the device according to claim 6, the method comprising: evaporating a refrigerant in the first tube and in the second tube, solidifying a liquid material on the first tube, on the second tube and on the fin.
 17. A method using the device according to claim 7, the method comprising: evaporating a refrigerant in the first tube and in the second tube, solidifying a liquid material on the first tube, on the second tube and on the fin.
 18. A method using the device according to claim 8, the method comprising: evaporating a refrigerant in the first tube and in the second tube, solidifying a liquid material on the first tube, on the second tube and on the fin.
 19. A method using the device according to claim 9, the method comprising: evaporating a refrigerant in the first tube and in the second tube, solidifying a liquid material on the first tube, on the second tube and on the fin. 